Engines of vehicles can operate with an emission control device to reduce exhaust emissions. In order to achieve efficient conversion of exhaust emissions, emission control devices such as catalytic converters require specific exhaust air-fuel ratios. Thus, fuel injection is typically controlled in order to provide a desired exhaust air-fuel ratio and thereby improve emission reduction.
However, the inventors herein have recognize a disadvantage with systems that transition valve timing between early intake valve closing (EIVC) and late intake valve closing (with valve closing past BDC (bottom dead center) (i.e., in the compression stroke). In particular, the transition between early intake valve closing (EIVC) operation and late intake valve closing (LIVC) operation can be difficult to manage.
For example, when the air-fuel charge starts to be pushed back into the intake manifold from the cylinder in late valve timing (due to compression from piston motion), the air-to-fuel mixture supplied to the subsequent cylinders is disturbed. This is because the air-fuel mixture is pushed back into the intake manifold. Deviations in the exhaust air-fuel ratio occur as a result and there can be a degraded impact on catalyst performance and tailpipe emissions. In other words, the air-fuel mixture pushed back into the intake manifold changes the air-fuel mixture to be drawn into subsequent cylinder events.